We measured black carbon (BC) with a seven-wavelength aethalometer (AE-31) at the Nam Co Lake (NCL), the hinterland of the Tibetan Plateau (TP) from May 2015 to April 2016. The daily average concentration of BC was 145 +/- 85 ng m(-3), increasing by 50% since 2006. The seasonal variation of BC shows higher concentrations in spring and summer and lower concentrations in autumn and winter, dominated by the adjacent sources and meteorological conditions. The diurnal variation of BC showed that its concentrations peaked at 9:00-16:00 (UTC + 8), significantly related to local human activities (e.g., animal-manure burning and nearby traffic due to the tourism industry). The concentration-weighted trajectory (CWT) analysis showed that the long-distance transport of BC from South Asia could also be a potential contributor to BC at the NCL, as well as the biomass burning by the surrounding residents. The analyses of the absorption coefficient and absorption angstrom ngstrom exponent show the consistency of sourcing the BC at the NCL. We suggest here that urgent measures should be taken to protect the atmospheric environment at the NCL, considering the fast-increasing concentrations of BC as an indicator of fuel combustion.

Light-absorbing organic aerosol (brown carbon (BrC)) can significantly affect Earth's radiation budget and hydrological cycle. Biomass burning (BB) is among the major sources of atmospheric BrC. In this study, day/night pair (10-h integrated) of ambient PM(2.5)were sampled every day before (defined as T1,n = 21), during (T2,n = 36), and after (T3,n = 8) a large-scale paddy-residue burning during October-November over Patiala (30.2 degrees N, 76.3 degrees E, 250 m amsl), a site located in the northwestern Indo-Gangetic Plain (IGP). PM(2.5)concentration varied from similar to 90 to 500 mu g m(-3)(average +/- 1 sigma standard deviation 230 +/- 114) with the average values of 154 +/- 57, 271 +/- 122, and 156 +/- 18 mu g m(-3)during T1, T2, and T3 periods, respectively, indicating the influence of BB emissions on ambient air quality. The absorption coefficient of BrC (b(abs)) is calculated from the high-resolution absorption spectra of water-soluble and methanol-soluble organic carbon measured at 300 to 700 nm, and that at 365 nm (b(abs_365)) is used as a general measure of BrC. The b(abs_365_Water)and b(abs_365_Methanol)ranged similar to 2 to 112 Mm(-1)(avg 37 +/- 27) and similar to 3 to 457 Mm(-1)(avg 121 +/- 108), respectively, suggesting a considerable presence of water-insoluble BrC. Contrasting differences were also observed in the daytime and nighttime values of b(abs_365_Water)and b(abs_365_Methanol). Further, the levoglucosan showed a strong correlation with K+(slope = 0.89 +/- 0.06,R = 0.92) during the T2 period. We propose that this slope (similar to 0.9) can be used as a typical characteristics of the emissions from paddy-residue burning over the IGP. Absorption angstrom ngstrom exponent (AAE) showed a clear day/night variability during the T2 period, and lower AAE(Methanol)compared to AAE(Water)throughout the sampling period. Further at 365 nm, average relative atmospheric radiative forcing (RRF) for BrC(Water)is estimated to be similar to 17%, whereas that of BrCMethanol similar to 62% with respect to elemental carbon, suggesting that BrC radiative forcing could be largely underestimated by studies those use BrC(Water)only as a surrogate of total BrC.

While brown carbon (BrC) might play a substantially important role in radiative forcing, an estimation of its light absorption contribution with high-time resolution is still challenging. In this study, a multi-wavelength (370-950 nm) Aethalometer was applied to obtain the wavelength dependent light absorption coefficient (sigma(abs)) of aerosols both before and after being heated to 250 degrees C. An improved absorption angstrom ngstrom exponent (AAE)-based method was developed to evaluate the contribution of BrC to light absorption at a wavelength of 370 nm (sigma(abs,BrC)/sigma(abs,370nm)). The sigma(abs,BC) at 370 nm was determined from the field measured AAE values for the wavelengths from 880 to 950 nm with a one-hour resolution. The simultaneous measurements of heated aerosols help confirm the negligible influence of BrC on the sigma(abs) values across the range of 880-950 nm. Meanwhile, sigma(abs,BrC)/sigma(abs,370nm) was also estimated with previously reported methods by assuming that the AAE was equal to 1 (Method I) as well as a new approach based on the light absorption enhancement (Method II). While the estimated sigma(abs,BrC)/sigma(abs,370nm) based on our developed method and Method I is highly correlated (r(2) = 0.78), the difference could be as large as > 20% on average. The obtained mean sigma(abs,BrC)/sigma(abs,370nm) was negative with Method II, a indicating the net production of BrC when the aerosols were heated. The difference between the values for sigma(abs,BrC)/sigma(abs,370nm) obtained by our developed method and by Method II was similar to 40% on average and much higher (> 50%) during the noon hour, when secondary organic aerosols and sulfate were abundant. We propose that it is more suitable to use an AAE around 0.7 for pure BC to evaluate the contribution of BrC to light absorption in the PRD region. The developed method thus helps improve our understanding of the light absorption and climate forcing of BrC.